1. Field of the Invention
This invention relates to the process of forming the production tooling for injection molding a one-piece reflective pavement markers, in particular, to methods of forming and positioning the precision patterns of the micro prismatic cube corner reflective elements which need to be incorporated within the molding apparatus to monolithically form a one-piece reflective pavement marker.
2. Related Art
Typical fabrication process for making a reflective pavement marker is based on two or three independent steps, each with its own molding or forming apparatus.
Reflective pavement markers are used as lane dividers or guardrail delineators. Other roadway markers are used as temporary lane dividers during roadway repairs.
Since 1965, the most commonly used retro reflective pavement markers are based on Heenan U.S. Pat. No. 3,332,327, Balint U.S. Pat. No. 3,409,344.
Typically, this type of reflective markers are produced in a process consisting of three to five steps: Firstly, injection molding of a thermoplastic shell-like housing, either integrally molded with the prismatic reflective faces utilizing molds with slides, or the reflective faces with its multiple, three dimensional cube corner reflective prisms are injection molded separately, using standard mold, then pre inserted in an injection molding process forming the thin shell-like housing. The shell-like housing firstly coated with metallic sealer to promote reflectivity of the prismatic faces, then filled with resinous epoxy filler material; alternatively, the shell like housing is agglutinated to a hollowed, polymeric structural body forming a reflective pavement marker.
Tooling for the prismatic cube corner reflective elements are positioned within one or two inclined, opposing faces within the core element of the shell like housing mold. Each face is having multiple cube corner reflective elements.
Typically, the core portion of the mold for forming the shell-like housing is provided with slides that move in a third axis, in addition to the x and y direction of the molding apparatus.
This type of molds provides adequate draft and allows the formation of the three cube corner surfaces of each reflective element within the reflective faces without obstruction but with slight deformity and inconsistency in its surface accuracy.
Therefore, variation in retro-reflectivity can be detected among various production patches.
In addition to molding the shell-like housing, a second mold is needed to form a hollowed thermoplastic structural body that would be agglutinated to the shell-like housing to complete a reflective pavement marker.
Alternatively, the shell like housing is sealed with a reflective metallic coat, then a resinous filler material, such as epoxy or urethane filled with inert filler material is used to fill the shell like housing to provide structural backing.
The core and cavity portions of the mold apparatus forming this shell-like housing typically are positioned exactly in the same x and y direction of the open and close direction of the injection machine. Mold slides are provided to offset drafts due to the three dimensional shapes of the cube corner prisms within this apparatus.
Specially electroformed molding portions that define the cube corner reflective elements are normally affixed onto the core segment of this molding apparatus.
While the methods and apparatus described in the afore-mentioned arts perform effectively in producing good quality reflective pavement markers, a disadvantage of such previous systems is the costly processes and the time involved in assembling the multi-pieces and metal coating the reflective elements of the reflective marker.
Another critical disadvantage in the present arts for forming reflective pavement markers is the inconsistency in forming a precision pattern for the micro-prismatic cube corner reflective elements. Such inconsistency is due to warping and shrinkage factors during the injection molding process that mold approximately an area of about 2.5 to 3.5 square inches with hundreds of cube corner reflective elements; each element with three surfaces perpendicular to each other. Such large surface area with about 0.10-inch thickness tends to have higher deformity due to shrinkage and/or cooling time. This warping reduces the collective retro-reflection of the cube corner reflective elements.
Accordingly, it is desirable to provide a method and apparatus that consistently produces a high quality reflective pavement marker. It is further desirable to provide such a method and apparatus that produce a monolithically formed one piece reflective pavement marker integrally including both, the cube corner reflective elements as well as the structural wall elements that minimize warping of the reflective face, due to dividing the face into small cell like regions.